U.S. patent number 7,939,987 [Application Number 12/256,494] was granted by the patent office on 2011-05-10 for acoustic wave device employing reflective elements for confining elastic energy.
This patent grant is currently assigned to Triquint Semiconductor, Inc.. Invention is credited to Benjamin P. Abbott, Robert Aigner, Julien Gratier, Taeho Kook, Marc Solal.
United States Patent |
7,939,987 |
Solal , et al. |
May 10, 2011 |
Acoustic wave device employing reflective elements for confining
elastic energy
Abstract
An acoustic wave device includes electrodes carried on a surface
of a piezoelectric material and an array of reflective obstacles
such that elastic energy resulting from a piezoelectric effect is
preferentially directed along a primary wave propagation path. The
array of reflective obstacles are positioned generally parallel to
the surface of the piezoelectric material and redirect acoustic
waves typically reflected in other than a desirable direction to
along a desired direction generally along the primary propagation
path. The obstacles improve performance for SAW and BAW devices by
effecting reflected energy and suppressing spurious modes.
Inventors: |
Solal; Marc (Longwood, FL),
Aigner; Robert (Ocoee, FL), Gratier; Julien (Apopka,
FL), Kook; Taeho (Orlando, FL), Abbott; Benjamin P.
(Longwood, FL) |
Assignee: |
Triquint Semiconductor, Inc.
(Orlando, FL)
|
Family
ID: |
43928282 |
Appl.
No.: |
12/256,494 |
Filed: |
October 23, 2008 |
Current U.S.
Class: |
310/313R;
333/195; 310/313D; 310/313B |
Current CPC
Class: |
H03H
9/02866 (20130101); H03H 9/02118 (20130101); H03H
9/02645 (20130101) |
Current International
Class: |
H03H
9/145 (20060101); H03H 9/25 (20060101) |
Field of
Search: |
;310/313D ;333/195 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Olli Homgren, Tapani Makkonen, Jouni Knuuttila, Mikko Kalo, Victor
Plessky, and William Steichen; "Side Radiation of Rayleigh Waves
from Synchronous SAW Resonators"; IEEE Transactions on Ultrasonics,
Ferroelectrics, and Frequency Control, vol. 54, No. 4, Apr. 2007;
pp. 861-869. cited by other .
Sarah Benchabane, Abdelkrim Khelif, William Daniau, Laurent Robert,
Valerie Petrini, Badreddine Assouar, Brice Vincent, Omar Elmazria,
Jan Kruger, and Vincent Laude; "Silicon Phononic Crystal for
Surface Acoustic Waves"; 2005 IEEE Ultrasonic Symposium; pp.
922-925. cited by other .
J. Kaitila, M. Ylilammi, Juha Ella, and R. Aigner; "Spurious
Resonance Free Bulk Acoustic Wave Resonators"; 2003 IEEE
Ultrasonics Symposium; pp. 84-87. cited by other .
Julius Koskela, Jouni Knuuttila, Tapani Makkonen, Victor Plessky,
and Martti Salomaa; "Acoustic Loss Mechanisms in Leaky SAW
Resonators on Lithium Tantalate"; IEEE Transactions on Ultrasonics,
Ferroelectrics and Frequency Control; vol. 48, No. 6; Nov. 2001;
pp. 1517-1526. cited by other.
|
Primary Examiner: SanMartin; J.
Attorney, Agent or Firm: Allen, Dyer, Doppelt, Milbrath
& Gilchrist, P.A.
Claims
That which is claimed is:
1. An acoustic wave device comprising: a piezoelectric material;
metallic electrodes carried by the piezoelectric material such that
a difference of potential between two adjacent electrodes generates
elastic energy by piezoelectric effect, wherein a primary wave
propagation of acoustic waves is along a primary wave propagation
path generally perpendicular to the metallic electrodes; and a
plurality of obstacles reflecting at least a portion of the
acoustic waves, the plurality of obstacles positioned generally
parallel to a surface of the piezoelectric material for confining
the elastic energy along the primary wave propagation path, wherein
obstacles within the plurality of obstacles are separated by a
preselected periodicity, and wherein at least one direction of
periodicity is parallel to the metallic electrodes.
2. An acoustic wave device according to claim 1, wherein the
preselected periodicity includes an integer number of a half
wavelengths of a preselected acoustic wave generated by the
metallic electrodes.
3. An acoustic wave device according to claim 1, wherein the
preselected periodicity does not exceed three elastic
wavelengths.
4. An acoustic wave device according to claim 1, wherein the
metallic electrodes form an interdigitated transducer (IDT) and the
obstacles form a two dimensional array of obstacles.
5. An acoustic wave device according to claim 4, wherein the
periodicity of the obstacles is essentially the same as the
periodicity of the electrodes of the IDT.
6. An acoustic wave device according to claim 4, wherein the
periodicity of the obstacles is sufficiently small to allow more
than 5 periods in an aperture of the IDT.
7. An acoustic wave device according to claim 4, wherein the IDT is
positioned between two reflectors so as to form a resonator having
periodic metallic electrodes, and wherein the array obstacles is
such that the periodicity of the obstacles is essentially the same
as the periodicity of the reflectors.
8. An acoustic wave device according to claim 1, wherein the
plurality of obstacles comprise a dotted metallic layer deposited
on a surface of the metallic electrodes.
9. An acoustic wave device according to claim 8, wherein the dotted
metallic layer is deposited on at least one of an upper surface and
a lower surface of the metallic electrodes.
10. An acoustic wave device according to claim 8, wherein the
obstacles comprise the dotted metallic layer having dots separated
on the electrodes by a least one dielectric layer.
11. An acoustic wave device according to claim 8, wherein the
dotted metallic layer comprises tungsten.
12. An acoustic wave device according to claim 1, wherein the
obstacles comprise strips of a dielectric material attached to at
least one of above and below the electrodes.
13. An acoustic wave device according to claim 1, wherein the
obstacles are strips of material separated from the electrodes by a
dielectric material.
14. An acoustic wave device according to claim 1, wherein the
obstacles are elongate grooves in a dielectric material carried on
an upper surface of the electrodes.
15. An acoustic wave device according to claim 1, wherein the
obstacles comprise at least one of an elongate groove within a
surface of the piezoelectric substrate, a strip of material
thereon, an etching therein, and a dot of material thereon.
16. An acoustic wave device according to claim 1, wherein the
obstacles comprise at least one of Molybdenum, Aluminum, Silicon
Nitride, Aluminum Nitride, and Silicon Oxide.
17. An acoustic wave device according to claim 1, wherein the
obstacles comprise grooves formed on the surface of the metallic
electrodes.
18. An acoustic wave device according to claim 1, wherein an
alignment of obstacle centers and an alignment of electrode centers
are shifted to provide unidirectionality.
19. An acoustic wave device according to claim 1, wherein the
electrodes and the plurality of obstacles form a SAW filter
comprising at least one of a SAW transducer and a resonator.
20. An acoustic wave device according to claim 1, wherein the
metallic electrodes and the plurality of obstacles form a bulk
acoustic wave (BAW) resonator, and wherein the plurality of
obstacles form a two dimensional array of obstacles carried
generally perpendicular to the primary wave propagation path.
21. An acoustic wave device according to claim 20, further
comprising a Bragg reflector isolating the BAW resonator from the
piezoelectric material and confining the energy in a first
direction corresponding to the thickness of a piezoelectric layer
and the two dimensional grating of obstacles for confining the
energy in other directions.
22. An acoustic wave device according to claim 1, wherein the
plurality of obstacles form an array of obstacles aligned along
first and second axes, and wherein a periodicity of obstacles
within the axes is interrupted by at least one obstacle being
missing from the array.
23. An acoustic wave device according to claim 22, wherein the at
least one missing obstacles provides at least one of a cavity and a
wave guide within the plurality of obstacles.
24. An acoustic wave device comprising: a piezoelectric substrate;
metallic electrodes carried by the piezoelectric substrate such
that a difference of potential between two adjacent electrodes
generates elastic energy by piezoelectric effect, wherein a primary
wave propagation is along a primary wave propagation path generally
perpendicular to the metallic electrodes; and a plurality of
obstacles operably reflecting acoustic waves, the plurality of
obstacles positioned generally within a two dimensional array,
wherein periodicities of obstacles within the array are along the
primary wave propagation path and perpendicular to the primary
propagation path.
Description
FIELD OF INVENTION
The present invention generally relates to the field of acoustic
wave devices using piezoelectric materials, and more particularly
to an acoustic wave device employing electrodes carried on a
surface of a piezoelectric material and reflective obstacles acting
as reflectors confining generated elastic energy.
BACKGROUND OF THE INVENTION
Elastic or acoustic waves are typically used for a long time to
design devices like resonators or filters. A well known example is
the quartz resonator using Bulk Acoustic Waves illustrated by way
of example with reference to FIG. 1. The principle is to use the
piezoelectric effect of quartz to transform electric energy to
acoustic energy inside a quartz crystal. Then, a resonance mode is
obtained at the frequency such that the thickness of quartz plate
is equal to half a wavelength. These devices are typically used at
relatively low frequencies (some kHz to some MHz).
At higher frequencies (GHz range), the same principle can be
applied. Since the wavelength is in the range of some micrometers,
the thickness of the piezoelectric material is now very thin. To be
able to manufacture these devices, thin film deposition techniques
have to be used. Typically, thin films are deposited on a wafer. To
confine the acoustic energy inside the piezoelectric film, the film
can be mechanically isolated of the substrate by air (membrane
approach) or reflective layers can be inserted between the
substrate and the film (Solidly Mounted Resonator) as shown on FIG.
2.
SAW devices use so-called interdigitated transducers at the surface
of piezoelectric substrates as illustrated with reference to FIG.
3. By piezoelectric effect, elastic waves are generated at the
surface of the substrate. For resonators, the transducer is placed
between two gratings. When the period of the grating is close to
half the acoustic wavelength, the waves reflected on the individual
electrodes are in phase and the grating reflects all the
energy.
In all the devices described before, the elastic energy is confined
for one direction. For example, for the quartz resonator or the
thin film resonator, the energy is confined in the thickness of the
piezoelectric film. For the SAW resonator, the energy is confined
between the two gratings in the direction perpendicular to the
electrodes. But for all these devices, it is difficult to confine
the energy in the other directions. For the quartz resonators or
the thin film resonator, the energy is confined below the
electrodes thanks to the mass of the electrodes but spurious modes
are often observed. In addition, some acoustic energy is often
radiated outside of the resonator resulting in losses. For the SAW
device, the lower velocity in transducers and gratings can result
in wave guiding the energy inside the active aperture, but again,
this also results in several modes and very often some energy is
radiated resulting in losses. For example, for SAW RF resonators
using Leaky SAW on Lithium Tantalate, it is known that acoustic
radiation occurs and generates loss. Such is described in [1] J.
Koskela, J. V. Knuuttila, T. Makkonen, V. P. Plessky, M. M.
Salomaa, "Acoustic Loss Mechanisms in Leaky SAW Resonators on
Lithium Tantalate", IEEE Trans. On UFFC, vol. 48, no6, November
2001, pp 1517-1526; and [2] O. Holmgren, T. Makkonen, J. V.
Knuuttila, M. Kalo, V. P. Plessky, W. Steichen, "Side Radiation of
Rayleigh Waves from Synchronous SAW Resonators", IEEE Trans. On
UFFC, vol 54, no 4, April 2007, pp 861-869.
For SAW filters on quartz, transverse modes occur often resulting
in ripple in the pass band or spurious emissions in the rejection
band. One technique to excite only one mode is to adapt the
velocity profiles in such a way that a mode having a flat amplitude
in the active region can exist. This mode is then matched to the
excitation profile meaning that it will be (almost) the only one
excited. This technique called "piston mode" can be applied for
both the BAW devices, as described in [3] J. Kaitila, M. Ylilammi,
J. Ella, and R. Aigner, "Spurious resonance free bulk acoustic wave
resonators", in Proc. IEEE Ultrasonics symp, 2003, pp. 84-87; and
for SAW devices, as described in [4] SAW FILTER OPERABLE IN A
PISTON MODE, U.S. application Ser. No. 11/863,479 filed Sep. 28,
2007.
Even though it is efficient to reduce the spurious modes, some
energy may still radiate outside of the resonators. It is desirable
to suppress both the spurious modes and the radiation loss. In
certain conditions, a grating having a two dimensional (2D) array
of periodically placed obstacles can reflect any incident elastic
wave with any incidence angle in a given frequency band. This is
referred to as a "phononic crystal" or "phononic band gap" effect.
The principle is that for a given incidence angle, the waves
reflected on obstacles are all in phase for a frequency for which
the period for this angle is half the wavelength (or a multiple of
half a wavelength). The reflection coefficient on the grating is
close to one inside a frequency band proportional to the reflection
coefficient on one individual reflective obstacle. When the
incidence angle varies, the center of this frequency band moves. If
the individual reflection coefficient is large enough, a frequency
band can exist when the reflection on the grating is close to one
for all incidence angles. These phononic crystals have the
potentiality to be used in a lot of devices and are very actively
studied. By way of example as described in [5] A. Khelif, A.
Choujaa, S. Benchabane, V. Laude and B. Djafari-Rouhani, "GUIDING
AND FILTERING ACOUSTIC WAVES IN A TWO-DIMENSIONAL PHONONIC
CRYSTAL", 2004 IEEE Ultrasonics symp. proc, pp 654-657, a
description is provided for possible applications of these devices
for low frequencies. By way of further example, the application of
phononic crystals to SAW devices is described in [6] Sarah
Benchabane et al., "Silicon Phononic Crystal for Surface Acoustic
Waves", 2005 IEEE Ultrasonics symp. proc. pp 922-925. In these
above described disclosures, very strong reflectors are used like
holes as elements of the crystal.
SUMMARY OF THE INVENTION
Features and advantages in accordance with the present invention
may be provided by an acoustic wave device according to the
teachings of the present invention may comprise a piezoelectric
material and metallic electrodes carried by the piezoelectric
material such that a difference of potential between two adjacent
electrodes generates elastic energy by piezoelectric effect,
wherein a primary wave propagation of acoustic waves is along a
primary wave propagation path generally perpendicular to the
metallic electrodes. A plurality of obstacles is positioned for
reflecting at least a portion of the acoustic waves and may be
positioned generally parallel to a surface of the piezoelectric
material for confining the elastic energy along the primary wave
propagation path, wherein obstacles within the plurality of
obstacles are separated by a preselected periodicity, and wherein
at least one direction of periodicity is parallel to the metallic
electrodes.
Yet further, the plurality of obstacles may comprise a two
dimensional periodic reflective obstacle array having periodicities
parallel to and perpendicular to the direction of acoustic wave
propagation. To confine the energy inside acoustic devices, one
embodiment of the present invention superposes a two dimensional
periodic grating on an elastic device, wherein energy is reflected
in all directions and for most directions will reduce energy loss
of the device and suppress the spurious modes.
Embodiments may comprise obstacle periodicities having an integer
number of half elastic wave wavelength. By way of example,
periodicities may be at most three elastic wavelengths. Metallic
electrodes may form an interdigitated transducer (IDT) and the
grating comprise a two dimensional grating of obstacles. The period
of the obstacles may be essentially the same as the period of the
electrodes of the IDT. The period of the obstacles may be
sufficiently small to allow more than 5 periods in a device
aperture. The IDT may lie between two reflectors so as to form a
resonator having periodic metallic electrodes, and wherein the
gratings comprise a two dimensional grating of reflective obstacles
such that the period of the obstacles are essentially the same as
the period of the reflectors.
A grating of obstacles may further comprise dotted metallic layer
deposited on a surface of the metallic electrodes. The dotted
metallic layer may be deposited on at least one of an upper surface
and a lower surface of the metallic electrodes. The grating of
obstacles may comprise the dotted metallic layer having obstacles
separated on the electrodes by a least one dielectric layer.
The obstacles may be made of stripes of a dielectric material
attached at least one of above and below the electrodes. Yet
further, the obstacles may be made of strips of material separated
from the electrodes by a dielectric material. The obstacles may
also be stripe shaped grooves in a dielectric material carried on
an upper surface of the electrodes. The obstacles are made of
stripe shaped grooves within a surface of the piezoelectric
substrate. The obstacles may comprise Molybdenum, Aluminum, Silicon
Nitride, Aluminum Nitride, or Silicon Oxide. The obstacles and the
electrodes may be shifted to provide unidirectionality.
The electrodes and obstacles may form a SAW filter comprising at
least one of a SAW transducer and a resonator. The transducer and
the two dimensional grating of obstacles may also form a bulk
acoustic wave (BAW) resonator. The BAW resonator may comprise a
Bragg reflector isolating the resonator from the substrate and
confining the energy in a first direction corresponding to the
thickness of a piezoelectric layer and the two dimensional grating
of obstacles to confining the energy in other directions.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention are described, by way of example, with
reference to the accompanying drawings in which:
FIG. 1 is a diagrammatical illustration of a known quartz resonator
circuit;
FIG. 2 is a diagrammatical illustration of a known solidly mounted
resonator;
FIG. 3 is a diagrammatical schematic illustration of a SAW
resonator;
FIG. 4 is a diagrammatical plan view of one SAW resonator according
to the teachings of the present invention illustrated by way of
example;
FIG. 5 is a diagrammatical plan view of a second SAW resonator
according to the teachings of the present invention illustrated by
way of further example;
FIGS. 6A and 6B are plots of phase and amplitude of impedance,
respectively, for one known resonator;
FIGS. 6C and 6D are plots of phase and amplitude of impedance,
respectively, for one resonator embodiment according to the
teachings of the present invention;
FIG. 7 is table for comparison of the resonator parameters for
sample resonators described with reference to FIGS. 3, 5 and 6;
FIG. 8 is a plot illustrating a simulated coupling factor for a
tungsten layer above aluminum and a period of 2 .mu.m;
FIG. 8A is a plot illustrating a simulated coupling factor for a
tungsten layer below aluminum and a period of 2 .mu.m;
FIG. 9 is a plot illustrating a simulated resonance quality factor
for a tungsten layer above aluminum and a period of 2 .mu.m;
FIG. 9A is a plot illustrating a simulated resonance quality factor
for tungsten layer below aluminum and a period of 2 .mu.m;
FIG. 10 is a plot illustrating a simulated anti-resonance quality
factor for a tungsten layer above aluminum including a period of 2
.mu.m;
FIG. 10A is a plot illustrating a simulated anti-resonance quality
factor for a tungsten layer below aluminum including a period of 2
.mu.m;
FIG. 11 is a diagrammatical cross section view of buried IDT
devices with improved energy confinement, a top view thereof being
similar to FIG. 4 or FIG. 5 herein described;
FIG. 12 is a partial diagrammatical illustration of one embodiment
according to the teachings of the present invention using grooves,
wherein the upper illustration is a cross section view
perpendicular to the electrodes, and the lower illustration is a
cross section view along one electrode;
FIG. 12A is a partial diagrammatical illustration of a buried IDT
device with improved energy confinement using grooves, a top view
being similar to the embodiments of FIG. 4 or FIG. 5 herein
described;
FIG. 13 is a diagrammatical illustration of a configuration using
tape obstacles, wherein the obstacles may be made by grooves or
material deposition on top of the oxide for buried IDTs, and by
dielectric deposition above or below the transducers or by grooving
the substrate below the transducers;
FIG. 14 is a diagrammatical example of a unidirectional transducer
with improved confinement;
FIG. 15 is a diagrammatical example of a SPUDT with improved
confinement;
FIG. 16 is a second diagrammatical example of a SPUDT with improved
confinement;
FIGS. 17 and 17A are a diagrammatical examples of a thin film BAW
resonator with improved energy confinement, wherein FIG. 17 is a
partial cross section view of the device and FIG. 17A is a plan
view of the electrode;
FIG. 18 is a partial perspective diagrammatical view of a resonator
according to the teachings of the present invention;
FIGS. 19A and 19B illustrate impedance and phase responses,
respectively, for a well known resonator; and
FIGS. 20A and 20B illustrate impedance amplitude and phase
responses, respectively, for the resonator of FIG. 18 as modified
according to the teachings of the present invention by addition of
strip shaped grooves as obstacles.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will now be described more fully hereinafter
with reference to the accompanying drawings, in which preferred
embodiments of the invention are shown. This invention may,
however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein. Rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art. Like numbers refer to like
elements throughout.
Referring now to FIG. 3 with regard to SAW resonators, the period
for the electrodes in the gratings and the transducers is close to
half of the wavelength at the resonance frequency. Therefore, the
reflected waves on the successive electrodes are in phase so that
the global reflection coefficient is close to 1 and that the energy
is confined in the resonator. This is the basic principle of a SAW
resonator, but works only for the wave propagating in the
horizontal direction. As described in the above references [1] and
[2] for leaky SAW resonators, some energy is radiating outside of
the resonator in the busbars. This energy radiation results in
degradation of the resonator Q. For SAW resonators, on quartz it is
known that transverse modes exist and can be problematic for filter
applications. To avoid these issues, reflective obstacles periodic
in the vertical (transverse) direction are added to the resonator
of FIG. 3.
As illustrated herein by way of example with reference to FIG. 4,
one SAW resonator 10 may comprise a transducer 12 carried on a
surface 14 of a piezoelectric substrate 16 between two electrode
reflectors 18, 20. The electrode reflectors 18, 20 and transducer
12 are structured as is typical in the art. However, according to
the teachings of the present invention, other acoustic wave
reflectors, herein referred to as obstacles 22, or as dots for
selected embodiments, are periodically placed in a transverse
direction 24 and longitudinal direction 26, so as to form an
obstacle array, and for the SAW resonator 10 added to each of the
electrodes 28. For the SAW device herein described by way of
example, the longitudinal direction 26 is a direction of
propagation for primary acoustic waves. The reflective obstacles 22
herein described have periodicities along the longitudinal
direction 26 (the primary SAW propagation direction) and
perpendicular to the primary SAW propagation, such as along the
transverse direct 24. The obstacles 22 may be added above the
electrodes 28 or below, as desired. One embodiment includes a
transverse period 30 of the obstacles 22 being relatively close to
a horizontal period 32 of the electrodes 28, but does not have to
be identical. All obstacles 22 in successive electrodes 28 may be
aligned in square grid pattern 34, as illustrated with reference to
FIG. 4 or with alternating shifted obstacles 36 from one electrode
28 to an adjacent electrode 28A producing an offset pattern 38 as
illustrated with reference to FIG. 5, by way of example.
With such a configuration using the obstacles or dots 22, the
energy generated inside the transducer 12 is reflected by the
obstacles 22 and energy radiation loss is reduced or suppressed in
directions other than the primary acoustic wave path along the
longitudinal direction 26.
To illustrate this, reference is made to FIGS. 6A-6C, where a
comparison of results for the resonator 10 with and without the
obstacles 22 is illustrated. In this case, the piezoelectric
substrate 16 is a rotated 48.degree. Y-cut X propagation Lithium
Tantalate. Tungsten is used as the material for the obstacles 22.
Alternatively and by way of further example, the dots or obstacles
22 may be formed from Molybdenum, Aluminum, Silicon Nitride,
Aluminum Nitride, and Silicon Oxide.
By way of example with continued reference to FIG. 5, the
transducer 12 and electrode reflector 18, 20 period 32 is 2.0
microns. The electrodes 28 are aluminum having a thickness of 3700
angstroms. Tungsten has been chosen as material for the obstacles
22. For the example herein presented, the obstacles 22 appear as
"dots on a top surface of the electrodes 28. The thickness of the
dots 22 is 300 angstroms while their period 30 is 2.2 microns. The
aperture 11 of the resonator 10 is 30 microns (i.e. 7.5
wavelengths), which is a relatively narrow aperture for a SAW
resonator. The obstacles or dots 22 are herein alternated in the
offset pattern 38.
As illustrated with continued reference to FIGS. 6A-6D, the
standard resonator has losses above the resonance frequency, while
the new resonator 10 phase curve is much closer to a desired
rectangular shape. All parameters are improved as illustrated with
reference to the table of FIG. 7. Similar results were obtained for
a wide range of transverse periods 30 for placement of the
obstacles 22. Desirable results corresponded to a relatively larger
dot width/vertical period ratio. This ratio was 0.6 for the
resonator 10 herein illustrated. This technique may be applied to a
variety of SAW devices with different variants. One variant places
the obstacles 22 below the electrodes 28. An approximate simulation
can be done by assuming that the tungsten layer has a uniform
thickness. FIGS. 8 to 10 illustrate the simulated coupling factor,
Q at resonance and Q at anti-resonance for a resonator having a
layer of Tungsten above Aluminum.
According to the simulation, when the tungsten is above the
aluminum, a desirable quality factor is obtained for an aluminum
thickness (h) lower than 3000 angstrom, corresponding to a
normalized thickness h/(2p) of 0.075 while the thickness hw of
tungsten is between 100 and 300 angstroms for the uniform layers.
In the case of dots, the thickness of the dots is divided by the
ratio of the dot vertical width on the vertical period. Assuming a
ratio close to 0.5, this corresponds for the dots to a normalized
thickness of tungsten hw/2p between 0.005 and 0.015. When the
tungsten is below the aluminum, all parameters are improved and the
quality factors are increasing when the tungsten thickness are
increasing.
The examples above use tungsten for the dots. This was chosen for
its high acoustic impedance leading to a desirable reflectivity.
The invention gives similar results with other materials. Materials
may include conductors or dielectrics. In the case where dielectric
materials are used, the obstacles 22 may be strips rather than
dots. In addition, instead of adding material to the transducer 12,
the surface 14 of the substrate 16 may be grooved. To efficiently
avoid energy radiated outside the active area of the device, dummy
electrodes 40, as illustrated with reference to FIGS. 4 and 5, may
be used. In addition, the dots or strips can extend into the
bus-bar 42.
The invention may include a variety of surface waves such as
Rayleigh waves, surface transverse waves, leaky waves or other
elastic waves that can be excited by interdigitated transducers. By
way of further example, the invention may be used for boundary
elastic waves propagating at an interface between two
materials.
The obstacles 22 do not have to be in contact with the electrodes
28. By way of example, if the transducer 12 is buried inside a
dielectric material 44 used to reduce the temperature drift of the
acoustic device, the obstacles 22 may be above the dielectric
material 44 as illustrated with reference to FIG. 11a. If the
thickness of the dielectric material is sufficiently small so that
some acoustic energy exists at the surface, then the obstacles will
act as reflectors. This is possible even if more than one layer of
material is above the electrodes as herein illustrated by way of
example. As previously described with reference to FIG. 4, these
obstacles 22 are periodic as well aligned in the transverse
direction and in the longitudinal (propagation) direction.
Alternatively, several structures will come to the mind of those
skilled in the art now having the benefit of the teachings of the
present invention. By way of example, similar desirable results are
obtained by selectively etching the substrate 16 (or dielectric
layer as illustrated with reference to FIG. 12A, if included) to
form grooves 46 as illustrated with reference to FIG. 12.
By way of further example, transducers 12 and resonators may
include periodically placed obstacles 22 formed as strips of
reflective material aligned generally perpendicular to the
electrodes 28. One embodiment may comprise tape shaped obstacles 22
to confine the energy, as illustrated with reference to FIG. 13.
With this implementation, one has to be careful not to short
circuit the transducers electrodes. For buried IDTs devices, the
obstacles 22 may be grooves or metal at the surface of the oxide.
If no oxide is present, a dielectric material can be deposited
above or below the electrodes. One embodiment may include etching
the substrate 16 before depositing the transducer 12.
For the above described embodiment illustrated with reference to
FIG. 4, by way of example, the obstacle (reflector) centers 22C may
be aligned with the electrode centers 28C. This need not be the
only structure. It is known to SAW designers that a shift between
reflections centers and transduction centers can be advantageous to
make a unidirectional transducer. This may be done by shifting the
center axis 22A of the obstacles 22 from the center axis 28A of the
electrodes 28, as illustrated with reference to FIG. 14. If the
reflection on the electrodes 22 is negligible compared to the
reflection of the obstacle 28, a shift of one eighth of the
wavelength will give a desired unidirectionality. Unidirectionality
may still be obtained by using a different shift in the electrode.
One embodiment of the present invention including a unidirectional
transducer with improved energy confinement is illustrated with
reference to FIG. 14. Filter responses may be improved by varying
the reflectivity along the transducers and in some cases having
regions of the transducers including a shift between reflection and
transduction corresponding to a backward unidirectionality. This is
provided by an appropriate arrangement of the obstacles.
With continued reference to FIG. 14, material above the electrodes
or below is used to provide the energy confinement. If this is used
in the buried IDT configuration, the obstacle may be placed above
the dielectric layers. Alternatively, grooves may be used.
It is known that a succession of reflective electrodes and non
reflective electrodes with an adequate shift allows designing
unidirectional transducers. These transducers are often called
SPUDT (single phase unidirectional transducers). An example of a
SPUDT device uses EWC (electrode width control) or DART
(distributed acoustic reflection transducer). The non reflective
electrodes have a period of a quarter wavelength. By way of
example, the energy confinement may be obtained by adding the 2-D
array of obstacles 22 only above or below the reflective electrodes
28R as illustrated with reference to FIG. 15.
FIG. 16 illustrates another example of a SPUDT where the obstacles
22 are placed on all the electrodes 28, 28R, unlike the embodiment
of FIG. 15. The obstacles 22 may be placed on (or below) all
electrodes including the non reflective electrodes. In this case,
in the longitudinal direction 26, the reflectivity of the obstacles
on the non reflective electrodes is zero (a quarter of wave length
of distance), while a reflectivity still exists in the other
directions.
Different effects can be obtained when removing one or several
obstacles 22 from within the 2-D grating array. By way of example
with reference again to FIG. 14, obstacle 22X may be removed. This
introduces defects in a crystal. By removing one obstacle in the
array, a cavity results. At a one frequency or well defined
frequency range, the energy is concentrated inside the cavity.
Removing one row, by way of further example, a row along the
longitudinal direction 26 results in a wave guide, wherein the
energy is traveling inside the missing row. It is also possible to
couple a waveguide and a cavity. In this case, the energy is
traveling in the waveguide except for a specific frequency where it
is concentrated in the cavity. Those of skill in the art will
appreciate similar approaches can be used for the obstacles, now
having the benefit of the teachings of the present invention.
By way of example for a BAW resonator 48, the energy may be
radiated outside its resonator resulting in degraded losses. An
approach according to the teachings of the present invention may be
used to reflect the energy and confine it inside the resonator 48.
A 2-D periodic grating of obstacles 22 may be introduced inside the
resonator. For example, the obstacles 22 may be added above one of
the electrodes 28. The period of the obstacles 22 is chosen to
reflect the modes leaking the most energy. The same approach may be
used for quartz resonators or thin film BAW devices. FIG. 17
illustrates one diagrammatical schematic of a BAW resonator 48 by
way of example. As earlier described, the BAW resonator 48 may
comprise a Bragg reflector 49 isolating the resonator from the
substrate 16 and confining the energy in a first direction 50 (a
primary wave direction for BAW) through the thickness of a
piezoelectric layer 52 and the two dimensional grating 54 of
obstacles 22 for confining the energy in directions other than the
primary wave propagation direction 50.
By way of example, test results have shown desirable results from
embodiments of the invention. The teachings of the present
invention were applied for resonators in one case where a
dielectric layer is above the transducer to reduce the temperature
sensitivity, as illustrated with reference to FIG. 18. The
substrate is a 128 degree cut Lithium Niobate substrate. The period
of the electrodes is 1.8 microns. The obstacles 22 herein formed as
grooves 22G. The transducers/gratings are made essentially of
copper. The metal thickness is about 1800 Angstroms. The electrodes
28 are buried inside Silicon Oxide and thickness of 7000 Angstrom
of Silicon Oxide is added on top of the transducer 12 (total
thickness of oxide is about 8800 A). To illustrate a desirable
result and satisfaction of a need in the art, FIGS. 19A and 19B are
presented to illustrate impedance amplitude and phase of a standard
resonator. Strong spurious effects due to transverse modes can be
seen on the responses and are herein identified using arrows. FIGS.
20A and 20B illustrate results obtained for the same resonator
producing the results of FIGS. 18a and 18b, when strip shaped
grooves are etched on top of the oxide orthogonally to the
electrodes. The period of the grooves is 6 .mu.m and their depth is
about 1000 A. It can clearly be seen from the responses that the
amplitude of the spurious effects due to the transverse mode is
desirably reduced as a result of the grooves.
Many modifications and other embodiments of the invention will come
to the mind of one skilled in the art having the benefit of the
teachings presented in the foregoing descriptions and associated
drawings. Therefore, it is understood that the invention is not to
be limited to the specific embodiments disclosed, and that
modifications and embodiments are intended to be included within
the scope of the appended claims.
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